Noob here... no power to uno r3

I have bought 4 sainsmart uno r3 kits (classroom projects)... they all worked initially but now two of them now have no power(green light), have tried swapping out the usb cable but didn't help. Have my students somehow blown the board? Should the arduino be disconnected from usb while building projects on breadboards? Any advise would be helpful.

Have you really plugged the Cable right into Uno? (sometimes it seems to be that they are plugged, but they are not and need a bit more pressure).

I can't believe that within one lessons 2 of 4 can get destroyed... (But i do not know what they probably did)

I usually plug in and out when the Arduino is connected to PC.